Here's how you do it.
k = 0.693/t1/2Substitute and solve for k. Substitute k into the below equation.
ln(No/N) = kt
No = atoms you start with.
N = atoms you end with
k from above
t = time.
The easy way to do this problem is to let No = 100 (but you can pick any number you like)
then N = 10 (or 0.1 x the number you picked for No.
Then solve for t.

I did this several years ago with Cu-64 which has a half life of about 13 hours. Since it took almost two days to get to me (by air) and another 12-14 hours to run the experiment, it had to be HOT HOT HOT when it left the manufacturer so it would be at least HOT when I started counting.